1. Field of the Invention
The present invention relates to a liquid crystal display device and, more particularly, to a liquid crystal display device including an optical member with a region using an optical component such as a light emitting diode (LED) used as a backlight source, namely, a light characteristics changing layer for changing a spectrum distribution and chromaticity coordinates of light emitted from a back light.
2. Description of the Related Art
In general, the liquid crystal display (LCD) is a typical flat display device displaying images by adjusting the transmittance of light such that it corresponds to an image signal. Because the LCD is not a self-emission element that emits light by itself, it needs a light source for providing light from a rear side of a liquid crystal display screen to visually display images.
A power source circuit for driving a light source including a lamp to emit light to a front liquid crystal panel from the rear side of a liquid crystal module (LCM) and an integral adjunct complex making planar light are called a backlight unit. According to a light emission method, the backlight unit is divided into two types: direct type backlight unit and an edge type backlight unit, and recently, the direct type backlight unit employing a surface light source such as an LED and the edge type flat backlight are also under study variably.
Here, as for the edge type backlight unit, its light source is positioned at the side of the LCD module, and light emitting from the light source is formed through a light guide plate. Such type of backlight can hardly avoid a phenomenon that luminance is generally degraded. Thus, in order to obtain uniformly distributed luminance, a more effective light guiding system, namely, a system for guiding light to a relatively far distance from the light source, is required, and an advanced optical technology is required to minimize a loss of light when light transmits from the light source to the relatively far distance.
FIG. 1 is an exploded perspective view of the related art edge-type LCD backlight unit, FIG. 2 is a sectional view of a general direct type LED LCD, and FIG. 3 illustrates a color region of an LED for a conventional LCD product.
As shown in FIG. 1, the edge type LED backlight unit includes a light source unit 10 providing light, a light guide plate 32 guiding light provided from the light source unit 10, and a diffusion sheet 34, a prism sheet 55, and the like, improving the optical characteristics of light outputted after transmitting the light guide plate 32.
Here, the light source unit 10 includes a substrate 12 and a plurality of LEDs 14. At this time, the LEDs 14 are mounted in a row on the substrate 12 and electrically connected with an external power source. The substrate with the plurality of LEDs 14 mounted thereon is inserted and fixed in a light source cover 16. A coupling groove 16a is formed along a longer axis direction of the light source cover 16 on an inner surface of the light source cover 16, and the substrate 12 is inserted in the coupling groove 16a so as to be fixed.
The light guide plate 32 guides light provided from the LED 14 of the light source unit 10 such that light is distributed to the entire upper surface of the light guide plate 32. A reflection plate 30 disposed below the light guide plate 32 increases light reflection efficiency.
The diffusion sheet 34 is disposed on the light guide plate 32 and uniformly distributes light that has transmitted through the light guide plate 32.
prism sheet 55 including first and second prism sheets 52 and 54 is disposed on the diffusion sheet 34. The prism sheet 55 guides light from the diffusion sheet 34 to a certain region, namely, to an image region, of a liquid crystal panel (not shown) on which images are displayed.
A protection sheet 56 may be disposed on the prism sheet 55 to protect the prism sheet 55 against an external scratch or the like.
The LCD device having the LED backlight unit needs a surface light source to implement image on its screen, and substantially, light of a dot light source form outputted from the LED package 14 is changed into a surface light source while passing through the light guide plate 32.
Meanwhile, the direct type backlight used for a large-scale liquid crystal display reflects light, which is outputted from a cold cathode fluorescent lamp (CCFL), from the reflection plate to the front liquid crystal panel. A milk-white scattering plate is disposed at a reflection path of the light to allow light of uniform illuminance (intensity of illumination) to be illuminated to the liquid crystal display. However, this method has a problem in that it makes the liquid crystal display an increased size due to the thickness of the backlight.
Recently, the backlight is made thinner and has an improved luminance by employing the LED, which is a two-pole element emitted only when current passes therethrough and has the characteristics of a fast response speed, low power consumption, semi-permanent light span, and the like, as a surface light source device. Above all, upon recognition that the LED can reproduce a natural color and high quality image compared with the conventional CCFL, solves the problem of a residual image of video, and environmentally friendly by not using mercury, the LED is good enough as a core component of the next-generation LCD that can replace the CCFL backlight unit.
With reference to FIG. 2, in the direct type LCD, based on a main support 70 formed as a mold product made of a synthetic resin or stainless steel and having a substantially rectangular frame shape, a backlight unit (not shown) on a lower cover 60 is mounted on a lower portion of the main support and a liquid crystal panel 90 is mounted on an upper portion of the main support, which are then fastened. An upper cover 99 fixing all the elements is assembled and fastened to the main support 70 and the lower cover 60, such that it covers the edges of the front surface of the liquid crystal panel 90.
First, direct type LED backlight providing light to the liquid crystal panel 90 includes an LED array including LEDs 66 mounted on the lower cover 60 and formed as at least one line to emit light, a plurality of printed circuit boards (PCBs) 64 with the LED array provided on its front surface and lighting the LED array by being driven up and down, a reflection plate 62 formed on a lower front surface of the region where the PCBs 64 are driven, a diffusion plate 82 provided at an upper portion of the LED array and reducing non-uniformity of light generated from the LED array, a prism sheet 84 provided on the diffusion plate 82, and a protection sheet 86 protecting the prism sheet 84 and increasing a viewing angle.
When the backlight unit completed, the main support 70 having the substantially rectangular frame shape and formed of the mold product of a synthetic resin or stainless steel is fastened, and then, the liquid crystal panel 90 is mounted thereon. Here, the liquid crystal panel 90 is fabricated by attaching a thin film transistor array substrate and a color filter substrate with liquid crystal injected therebetween.
For example, in generating white light by mixing a blue chip providing light of blue wavelength and yellow phosphor excited by the light provided from the blue chip in order to provide white light from the backlight unit of the edge type or the direct type LCD device, as shown in FIG. 3, only a portion of the entire color available range of the LED manufactured for the LCD device is limited in producing the LEDs due to the chrominance coordinates standards and the like. In this case, an LED out of the available range of the plurality of LEDs is inevitably produced, so handling of such LED out of the available range causes an increase in the LED unit cost.
In other words, in order to adjust color viewing sensation to a desired level while satisfying chromaticity coordinates, the LED out of the available range should come within the available region, for which, in the related art, various changes are sought in relation to color filters or the like, which, however, is not easy.
Namely, various types of color filters have been developed and manufactured correspondingly one by one according to the chromaticity coordinates region of the LED, incurring considerable costs for the color filter investment and development.